1. Field of the Invention
This invention relates to a heat conductive boron nitride filler and an electrically insulating/heat dissipating sheet using the same.
2. Background Art
Electrically insulating/heat dissipating sheets are generally prepared by mixing an organopolysiloxane, a heat conductive inorganic powder and a crosslinking agent, and shaping the resulting silicone rubber composition into a sheet form, followed by vulcanization. They are used for the electrical insulation and heat dissipation of heat-generating electronic parts such as power transistors and thyristors. Exemplary of the heat conductive inorganic powder are hexagonal boron nitride (often abbreviated as BN, hereinafter), alumina, beryllia and diamond. Of these, BN is most preferable when a total profile of properties, cost and safety is taken into account. However, BN particles are of a layered crystal structure and have a tabular shape. The heat transfer of tabular BN particles largely differs with direction. Illustratively, the heat transfer in a direction parallel to the layer (referred to as a-axis direction, hereinafter) is higher than the heat transfer in a direction perpendicular to the layer (referred to as c-axis direction, hereinafter) by a factor of about 30. Therefore, in order to improve the heat transfer of electrically insulating/heat dissipating sheets in the thickness direction, tabular particles of BN are preferably distributed such that their a-axis is aligned in the thickness direction of the sheet. However, the electrically insulating/heat dissipating sheets must be finished to a smooth surface for ensuring close contact with heat-generating electronic parts and heat sink fins. In a common practice used to this end, a silicone rubber composition loaded with BN powder is formed into a sheet, followed by press vulcanization. In the sheeting step, BN particles tend to orient parallel to the sheet surface. Since the heat-dissipating route of the sheet is given in the c-axis direction of BN particles, this orientation means that the sheet fails to take advantage of the enhanced heat transfer of BN.
Thus, various attempts were made in the art for orienting BN particles such that their a-axis is aligned perpendicular to the sheet surface. For example, JP-A 244094/1996 discloses a method involving extrusion molding a kneaded compound into a plurality of plasticized bands, and integrating them with the aid of lips. In JP-B 38460/1994, a once press molded sheet is cut into bands, and one band is joined to a 90.degree. rotated band to form a sheet. Either of these methods requires extra steps or installations, which are disadvantageous from the standpoints of productivity and cost.